Lowering your car by getting shorter tires?

[cfg83]

blackjackel -

Originally Posted by blackjackel

...

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/65-15 5.0in 12.5in 25.0in 78.5in 807 0.0%
185/70-14 5.1in 12.1in 24.2in 76.0in 834 -3.1%

The new tire would be .4 inches or 10 millimeteres narrower, or approx 1.5 inches if you count all tires combined.

I'm going from a GoodYear integrity to BridgeStone B381
This is going from a rolling resistence of 0.00955 to a RR of 0.00615 or a difference of 0.00340 in rolling resistence!

I expect at least 5 more MPG, if not more.

Any thoughts?
__________________

Subtracting 3.1% implies losing MPG to me. I wouldn't go with a tire size below spec on the drive wheels. But who knows? It could make a good experiment. Do you EOC? It won't matter when the engine is off.

If you can, try to do a before/after GPS correction. I did the after, but I wish I had done the before. I know I am getting a 2% gain right now, but I don't know if I was already +X% to begin with.

CarloSW2

[blackjackel]

Originally Posted by cfg83

blackjackel -



Subtracting 3.1% implies losing MPG to me. I wouldn't go with a tire size below spec on the drive wheels. But who knows? It could make a good experiment. Do you EOC? It won't matter when the engine is off.

If you can, try to do a before/after GPS correction. I did the after, but I wish I had done the before. I know I am getting a 2% gain right now, but I don't know if I was already +X% to begin with.

CarloSW2

why wouldn't you go below spec on the drive tires? just so you wouldn't screw with the odometer?

Also, don't you think the reduction in wheel contact with the road coupled with the decreased rolling resistence of the B381's would well outdo the loss in mileage due to increased rpms? I would only have to do 27 more revolutions per mile....

And lastly, since you're in my area (I'm in west L.A.) would you like to get together to work on modifying our cars for better aerodynamics... I really don't know anyone who's willing to work with me on this. I realize it's an odd suggestion and I understand if you're weary about meeting a stranger from online

[cfg83]

blackjackel -

Originally Posted by blackjackel

why wouldn't you go below spec on the drive tires? just so you wouldn't screw with the odometer?

Also, don't you think the reduction in wheel contact with the road coupled with the decreased rolling resistence of the B381's would well outdo the loss in mileage due to increased rpms? I would only have to do 27 more revolutions per mile....

You know, I thought about it and I don't have a good answer. I think I am biased in favor of "taller gearing" wherever possible. You are coming from the LRR + better Aerodynamics POV. I do know that some Lexus models have a suspension "auto-lowering" feature at highway speed, so you are emulating that.

For LRR, I think we should ask CapriRacer a question. I want to see a before(current tires)/after(proposed tires) of what the tread looks like. We can get "pristine pictures" of the tire tread on TireRack. I will post on CapriRacer's tire thread later tonight for details of what I mean.

And lastly, since you're in my area (I'm in west L.A.) would you like to get together to work on modifying our cars for better aerodynamics... I really don't know anyone who's willing to work with me on this. I realize it's an odd suggestion and I understand if you're weary about meeting a stranger from online

Sending you a PM ...

CarloSW2

[Big Dave]

I've extensively studied this on pickups. choosing the optimum tire size is something the factory boys generally get right.

Remarks refer to driving wheels.

Go up in diameter and you lose MPG (even after recalibrating your speedo). Larger tires have a larger rotational moment of inertia and you have to expend more energy to accelerat them up to speed. Not a big deal if you exclusively drive the slab, but murder in town.

In a perfect world you could use the increased rotational energy in your bigger tires to coast further, but there is way too much traffic in my world for that.

Go down in diameter and your engine sppeds up and has to expend more fuel to overcome engine frictional HP.

With my 4x2, I split the difference. Kept the rear (drive) tires the stock size (235-85x16E) and reduced the front tires to 225-75x16E to lower the front and additional inch.

[Bror Jace]

I too am biased towards taller tires creating a longer (lower number) final drive ratio.

However, Big Dave, you are assuming that if you go up in tire size, you go up in mass. That isn't always true ... or at least not significant. Often, if you go "negative 1" (the opposite of "plus 1" sizing), like what blackjackal is proposing, you are going with a smaller diameter rim ... which is usually lighter.

[JohnnyGrey]

I too am biased towards taller tires creating a longer (lower number) final drive ratio.

Me 3.

[thebrad]

Originally Posted by blackjackel

After much much much study, research, and deliberation, the results of which can be found here ( http://ecomodder.com/forum/showthrea...ling-2813.html ) I have decided to get new wheels and tires. My tires have another 5,000 miles on em, 10,000 if I'm really really lucky.... so.. I will be doing the following:

from 15" wheels to 14" wheels.

Tire size from 195-65-15 to 185-70-14

I used this tire calculation site to calculate the changes in measurements, its not much at all: http://www.miata.net/garage/tirecalc.html

my odometer would be off by .9% (if it says i'm going 60 MPH i'll be really going 60.5 MPH)

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/65-15 5.0in 12.5in 25.0in 78.5in 807 0.0%
185/70-14 5.1in 12.1in 24.2in 76.0in 834 -3.1%

The new tire would be .4 inches or 10 millimeteres narrower, or approx 1.5 inches if you count all tires combined.

I'm going from a GoodYear integrity to BridgeStone B381
This is going from a rolling resistence of 0.00955 to a RR of 0.00615 or a difference of 0.00340 in rolling resistence!

I expect at least 5 more MPG, if not more.

Any thoughts?
__________________

Not possible, well it is "possible" but it's not worth it.
You cannot go "negative 1" on a 9th generation Toyota Corolla, unless you plan to only use 14s on the rears because the front discs won't allow for anything smaller than 15s. Even then it's really questionable because you're going to spend X amount of dollars on new 14x5" rims, then being unable to effectively rotate the tires you'll be going through tires a lot quicker. I don't think it will pay itself off fast enough to warrant the headaches.

[cfg83]

thebrad -

Originally Posted by thebrad

Not possible, well it is "possible" but it's not worth it.
You cannot go "negative 1" on a 9th generation Toyota Corolla, unless you plan to only use 14s on the rears because the front discs won't allow for anything smaller than 15s. Even then it's really questionable because you're going to spend X amount of dollars on new 14x5" rims, then being unable to effectively rotate the tires you'll be going through tires a lot quicker. I don't think it will pay itself off fast enough to warrant the headaches.

So the brakes require 15" rims, huh? That never occurred to me. That sounds like a "Toyota Corolla" forum question. I know about www.saturnfans.com for my car, but I don't know about a Toyota Corolla forum where I could ask a question like this.

CarloSW2

[thebrad]

Originally Posted by cfg83

thebrad -



So the brakes require 15" rims, huh? That never occurred to me. That sounds like a "Toyota Corolla" forum question. I know about www.saturnfans.com for my car, but I don't know about a Toyota Corolla forum where I could ask a question like this.

CarloSW2

Yeah the front rotor is 10.8" + 4" for the caliper, you need a 15" rim. I used to own a 2004 Corolla but totaled it in fall of 2007. 9thgenCorolla.com is the forum/resource for 2003-2009 Corollas. I also wouldn't recommend downsizing the brakes on that car since they are already somewhat underpowered.